This invention relates to a transistor circuit constructed by a combination of bipolar transistors and FETs (field effect transistors) which include IGFETs, MOSFETs, JFETs, etc.).
In an IC (integrated circuit) of the prior art, differential amplifiers shown in FIG. 1 are used widely. The arrangement of FIG. 1 includes a differential pair of transistors 1 and 2, a transistor 3 acting as a constant current source, a bias power source 7 for the differential pair of transistors 1 and 2, and a bias power source 8 as bias supply means for the transistor 3. The amount of the constant current of the transistor 3 is determined by a voltage of the bias power source 8 and a value of a resistance 6. The numerals 4 and 5 are resistances by which current variations of the differential pair of transistors 1 and 2, responding to the voltage variation of an input power source 14 connected between input terminals 9 and 10, are respectively supplied to output terminals 11 and 12 as a voltage variation. The numeral 13 designates a power supply terminal.
Referring to FIG. 1, if the output impedance of the constant current transistor 3 is very much larger than the active resistances of the transistors 1 and 2, the output impedances of (the transistors 1 and 2 at the emitter side) so that the constant current transistor 3 can be considered as an ideal constant current source supplying a constant current I.sub.0, the currents I.sub.1 and I.sub.2 of the differential pair of transistors 1 and 2 are respectively shown as the following formulas: ##EQU1##
In these formulas, k is the Boltzmann constant, T is an absolute temperature, q is a charge of an electron and V is a voltage signal of the input power source 14. Accordingly, the current variation .DELTA.I of the transistor 1 is approximately shown by the following formula: ##EQU2##
As can be seen from the above formula, the current variation .DELTA.I does not contain the even-number higher harmonics. As the result, the output signal of the differential amplifier has less distortion than a common-emitter transistor amplifier. As mentioned above, the differential amplifier has the capability of suppressing only the even-number higher harmonics so that a combination of the differential amplifiers is used widely as a signal amplifier circuit in ICs.
Although the above discussion holds true in theory, in actual practice the output impedance of the transistor 3 is finite. Because of this, the differential amplifier cannot cancel the even-number higher harmonics perfectly. Namely, in general, an effective base width of a transistor varies in response to the voltage between the collector and the emitter thereof, so that the collector current and the emitter current thereof vary, too. The output impedance of the transistor 3 is not much larger than the output impedance of the transistors 1 and 2 at the emitter side, that is, the active resistance r.sub.e, so that a variable current of transistor 1 partially flows into the collector of the transistor 3, and all of the variable current does not flow into the transistor 3. For this reason, the differential amplifier shown in FIG. 1 does not sufficiently decrease the even-number higher harmonics, and the output signals at the output terminals 11 and 12 contain the evennumber higher harmonics.
Enlarging the base width of the transistor is effective for decreasing the variation of the collector current responding to the voltage variation between the collector and the emitter. In this case, however, a current amplification factor (abbreviated hfe) drops down so that the transistor may be unable to function satisfactorily as a current amplification device. In general, the differential amplifiers are used as a combination as shown in FIG. 2, and the decrease of the hfe of the transistors may have a very bad influence.
In FIG. 2, the same reference numerals are used to denote the same members as those shown in FIG. 1. Further, transistors 38 and 39 are emitter-follower transistors corresponding to the bias power sources 7 and 8 of FIG. 1, resistances 17, 18 and 19 determine bias voltages of the differential pair of transistors 1 and 2 and the transistor 3, diode-connected transistors 36 and 37 are used for the temperature compensation of the voltages between bases and emitters of the transistor 3, transistors 32, 33 and 26 acting as constant current sources, and the emitter-follower transistor 39. Resistances 20 and 21 are ones for respectively biasing the bases of the differential pair of transistors 1 and 2, and transistors 30 and 31 are emitter followers to which the constant currents are respectively supplied from the constant current transistors 32 and 33. A differential pair of transistors 34 and 35 constitutes a differential amplifier with the constant current transistors 26. The transistor 39 acts as a common bias source to the constant current transistors 3, 32, 33 and 26. The numeral 40 designates a capacitance for blocking a direct current, and the numeral 41 denotes an input signal source.
In the above-mentioned configuration, enlarging the base width to decrease the variations of the collector currents and the emitter currents responding to the voltage variations between the collectors and the emitters of the constant current transistors 3, 32, 33 and 26 results in the decrease of the hfe as further mentioned. Therefore, the variation of the base current, which is 1/hfe of the variation of the collector current and the emitter current, becomes large in proportion to the degree that the hfe becomes small. Further, as the input signal 41 from the source is amplified by the differential amplifier formed by the transistors 1, 2 and 3, the variations of the collector voltages of the constant current transistors 32, 33 and 26 become large so that the variations of the base currents thereof become large. These variations of the base currents cause a variation of the emitter current of the transistor 39. As a result, the variation is input to the transistor 3. Namely, the output signal is partially fed back to the input.
If the differential pair of transistors 1 and 2 has a complete balance of the direct current and the alternating current, voltage signals generated at the resistances 4 and 5 have the same polarity and the same amplitude, so that there is no influence on the base input signal of the transistor 3. In general, however, the differential pair of transistors 1 and 2 does not have a balance, and the differential voltage thereof is amplified by the differential amplifier constructed by the differential pair of transistors 34 and 35 and the constant current transistor 26, so that the possibility of an oscillation becomes high. The differential pair of transistors 1 and 2 does not have a balance in principle, since input impedances of the differential pair of transistors 1 and 2 are respectively an impedance of the input signal source 41 and the resistance 21. So, some feedback certainly occurs and the possibility of the oscillation becomes very high.
As mentioned above, decreasing the distortion causes the circuit to oscillate, so that designing the circuit requires special consideration. In particular, emitter resistances 6, 24, 25 and 27 are connected to the emitters of the constant current transistors 3, 32, 33 and 26 to decrease the gains of those transistors. The effect of the emitter resistances 6, 24, 25 and 27 is described hereinafter.
In the arrangement shown in FIG. 2, as the output signal of the output terminal 11' is supplied to an output terminal 65 via an emitter-follower stage having a low output impedance formed by a transistor 66 and a resistance 67, a signal current partially flows into a ground-line resistance 59, so that a signal voltage occurs at the resistance 59. In general, an IC has several ground-lines because of lay-out restrictions. Therefore, as shown in FIG. 2, a ground-line of the transistors 36, 37 and 38 is separate from other ground-lines. As a result, an unnecessary signal voltage occurs between the base and the emitter of the transistor 3. Therefore, it is necessary to insert the emitter resistance 6 into the emitter of the transistor 3 in order to decrease the gain and to prevent an oscillation. The emitter resistances 24, 25 and 27 have the same function. Namely, the arrangement of FIG. 3 needs extra resistances to avoid undesirable oscillation.